In the earlier application, the coolant was infused with bubbles in the passage itself, at a surface of the wall of the passage which extended generally parallel to the flow of the coolant in the passage and coterminated with the exit end of the mold at the aperture to form an edge thereof. Moreover, as explained in the earlier application, the coolant was preferably infused with bubbles from a body of solid, but porous, gas-permeable material which extended in a continuous band around the wall of the passage, so as to maximize the area over which the gas was infused into the coolant flow. That is, it had been observed that the greater the area over which the bubbles were nucleated into the coolant, the finer the bubbles that were entrained in the flow, and the finer the bubbles, the less the tendency of the bubbles to coalesce and produce a massive rush of bubbles or "blow-out." Now, it has been observed still further that when finer bubbles are generated, such as from a continuous band, the coolant actually can be infused with bubbles at a much earlier location than in the passage itself, such as in an annular retention chamber that is circumposed about the exit end opening of the mold in the body thereof, and operable to charge the passage with the coolant that is discharged onto the surface of the ingot from the passage. This earlier location has the distinct advantage that even when the passage is in the form of a series of spaced holes that are arranged in an annulus around the exit end opening of the mold, the body of the porous material can still take the form of a continuous band of the same, if desired, because the body of material is disposed ahead of the holes, i.e., in such a retention chamber.